Stationary reciprocating engines operating on natural gas or other gaseous fuels use energy for ignition from a spark or from a small pilot quantity (typically five percent of the total fuel) of a liquid fuel having an adequate cetane number (typically diesel fuel oil) injected directly into the combustion chamber. The pilot ignited engines serve the major industrial markets since they exceed the spark ignited engines in durability and rating capability and offer convertibility to and from full diesel fueling while in operation. These pilot ignited engines are referred to as "gas diesel" or "dual fuel" engines.
Although the pilot ignited engine represents the most fuel efficient prime mover in commercial use at this time, these engines exhibit objectionable levels of exhaust emissions which are detected by analytical procedures and observed visually as yellow in color. Extensive work at the research level has related the objectional emission levels to the competition of the liquid pilot fuel and the primary gaseous fuel for available oxygen. This competition for oxygen favors the gaseous fuel bulk and starves the fraction of the pilot fuel resulting in high levels of emissions from the liquid fuel.